One reason is that current implementations do a great deal of cost shifting from the young and/or healthy to the older and/or sicker. With Vitality programs a low cholesterol healthy slender non-smoker pays less each month for health insurance than the average employee. (A "3000 point" gap -- enough to qualify for @$1000+ yearly insurance cost reductions.)

Horwitz, Kelly et al ....Our evidence suggests that savings to employers may come from cost shifting, with the most vulnerable employees—those from lower socioeconomic strata with the most health risks—probably bearing greater costs that in effect subsidize their healthier colleagues. ..

Is that enough of a shift to make wellness programs covertly cost-effective? Or are corporations just being irrational? I suspect a bit of both, but we gotta remember that Walmart memo. At the very least, they shift costs from the blessed to the unblessed.